In recent years, a motor of a motor unit for use in an automobile, such as an electric vehicle or a hybrid vehicle, has been required to satisfy the following conditions. That is, the motor requires quietness to provide comfort in the interior of an automobile. The motor is also required to be compact, high powered, and highly efficient since it is mounted in an automobile or a limited space. An electric vehicle, a hybrid vehicle, and the like are referred to as an electric vehicle (EV) in the following description.
Also in the following description, a motor unit for use in an automobile refers to a motor unit other than for a wheel drive motor for driving the automobile. The motor unit for use in an automobile includes, for example, a cooling fan to cool a battery, an electric pump to control hydraulic pressure, and an actuator to vary the blowing air volume of air conditioning. These motor units may be collectively referred to as an accessory.
Incidentally, inverter control is known as a method of driving a motor. Pulse width modulation (PWM) control is a type of inverter control. When a motor is driven by PWM control, a low carrier frequency reduces the increase in the amount of heat due to switching losses in the motor. A low carrier frequency also reduces the decrease in energy efficiency when the motor is driven.
Meanwhile, a wheel drive motor for driving EV changes, for example, its output power when operated by a driver. Behavior of the wheel drive motor, that is, the operating condition of the wheel drive motor is apparent to an occupant of EV. Thus, the occupant of EV can tolerate the operating sound emitted from the wheel drive motor to some extent.
The wheel drive motor can be driven by PWM control where the carrier frequency can be changed. When the wheel drive motor is driven by PWM control, the carrier frequency may be switched to a low frequency in an audible range depending on the operating condition of the wheel drive motor. When the carrier frequency is switched to the low frequency in the audible range, the wheel drive motor achieves both efficiency improvement due to the reduction of switching losses and improvement of comfort due to the reduction of noise.
Prior art documents in this field include, for example, PTL 1.